In recent years, in an image display device, such as a color liquid crystal display, enhancement in performance involves an increase in power consumption. In particular, for example, in a color liquid crystal display, enhancement in definition, expansion of a color reproduction range, or an increase in luminance involves an increase in power consumption of a planar light source device (backlight). In order to solve this problem, a technique in which each display pixel has a four-subpixel configuration with, for example, a white display subpixel displaying white in addition to three subpixels including a red display subpixel displaying red, a green display subpixel displaying green, and a blue display subpixel displaying blue, thereby improving luminance using the white display subpixel is attracting attention. With the four-subpixel configuration, since high luminance is obtained with the same power consumption as in the related art, if the luminance is the same as in the related art, it is possible to reduce power consumption in the planar light source device and to improve display quality.
For example, a color image display device described in Japanese Patent No. 3167026 has a unit which generates three kinds of color signals from an input signal using an additive primary color process, and a unit which adds the color signals of the three hues at an equal ratio to generate an auxiliary signal, and supplies four kinds of display signals in total including the auxiliary signal and three kinds of color signals obtained by subtracting the auxiliary signal from the signals of the three hues to a display unit. A red display subpixel, a green display subpixel, and a blue display subpixel are driven by the three kinds of color signals, and a white display subpixel is driven by the auxiliary signal.
Japanese Patent No. 3805150 describes a liquid crystal display which includes a liquid crystal panel in which a red output subpixel, a green output subpixel, a blue output subpixel, and a luminance subpixel form one main pixel unit such that color display can be performed. The liquid crystal display has a calculation unit which calculates a digital value W for driving the luminance subpixel and digital values Ro, Go, and Bo for driving the red output subpixel, the green output subpixel, and the blue output subpixel using digital values Ri, Gi, and Bi of a red input subpixel, a green input subpixel, and a blue input subpixel obtained from an input image signal. The calculation unit calculates the digital values Ro, Go, Bo, and W which satisfy the following relationship and with which enhancement in luminance from a configuration, in which only the red input subpixel, the green input subpixel, and the blue input subpixel are provided, is achieved with the addition of the luminance subpixel.Ri:Gi:Bi=(Ro+W):(Go+W):(Bo+W)
On the other hand, according to the technique described in Japanese Patent No. 3167026 or Japanese Patent No. 3805150, while the luminance of the white display subpixel is increased, the luminance of the red display subpixel, the green display subpixel, and the blue display subpixel is not increased. In order to solve this problem, for example, JP-A-2010-033014 describes a method of driving an image display device, in which each pixel has a first subpixel displaying a first primary color, a second subpixel displaying a second primary color, a third subpixel displaying a third primary color, and a fourth subpixel displaying a fourth color. According to this method, an expansion coefficient α0 is calculated, and an output signal is obtained on the basis of the concerned expansion coefficient α0.